On Three-Dimensionality of Turbulent Buoyant Channel Flow

A three-dimensional buoyancy-driven turbulent boundary layer is generated in fully developed turbulent channel flow. Setting the buoyancy forces perpendicular to the mean flow direction and using high Grashof number result in strong cross flow which is expected to have a similar influence to that of the previously studied pressure- and shear-driven threedimensional turbulence boundary layers (3DTBLs). The present buoyancy-driven configuration has the advantage of being in equilibrium and three-dimensional from inception. Few issues are presented and discussed in order to give better understanding of the underlying physics associated with buoyancy-induced 3DTBL flows. The characteristics of the resulting 3DTBL are examined in comparisons with the available data in the literature. Reduction of the structure parameter A1 (defined as the ratio between the total Reynolds shear stress to twice the turbulent kinetic energy TKE) is noticed in the edge of the inner layer which is attributed to the reduction of both Reynolds shear stress and turbulent kinetic energy. Explanation for such reductions is given by examining the changes in coherent structures due to buoyancy effect. Comparisons of some relevant quantities used in turbulence modeling with those of 2DTBL flow reveal the difficulty of establishing a universal model for a satisfactory 3DTBL flow prediction.